The present invention is directed at decreasing the potential for incendiary discharges caused by electrostatic charges that can accumulate on flexible containers. More particularly the present invention is directed toward decreasing the potential for incendiary discharges caused by electrostatic charges in flexible containers such as flexible intermediate bulk containers.
It has been found that the shifting of specific materials within containers made of woven fabrics, as well as particle separation between the materials and such containers during loading and unloading of the container cause triboelectrification and create an accumulation of static electricity on the container walls. In addition, the accumulation of static electricity is greater at lower relative humidity and increases as the relative humidity drops. Also, highly charged material entering such containers can create an accumulation of static electricity on the container walls. Electrostatic discharges from a charged container can be incendiary, i.e. cause combustion in dusty atmospheres or in flammable vapor atmospheres. Moreover, discharges can be quite uncomfortable to workers handling such containers.
One conventional approach to solving this problem is to use a grounded container. Such a container may include conductive fibers that are electrically connected to ground to carry the electric energy out of the bag. The use of a grounded container, however, works only as long as the container remains grounded. If the container becomes ungrounded, its ability to decrease the potential for an incendiary discharge is lost, and due to the higher capacitance of the conductive system, the discharge can be much more energetic and incendiary than conventional non-conductive containers. Additionally, fabrication of the conductive containers requires specialized construction techniques to ensure all conductive surfaces are electrically connected together for a ground source.
Another conventional approach to decreasing the potential for incendiary discharges in flexible containers has been directed toward decreasing the surface electrostatic field of the container. If the magnitude of the electrostatic field on the surface of a container is above a certain threshold level, the potential for an incendiary discharge due to the electrostatic charge exists. That threshold level is about 500 kilovolts per meter (kV/m) for intermediate bulk containers made from woven polypropylene fabric. By decreasing the surface electrostatic field below about 500 kV/m, the potential for an incendiary discharge is greatly decreased and believed to be rendered virtually non-existent. Attempts at reducing the surface electrostatic field level below about 500 kV/m have not, however, proven successful without proper grounding.
One such effort at decreasing surface electrostatic fields has focused on the creation of corona discharges. There are four basic types of electrostatic discharges: spark discharges; brush discharges; propagating brush discharges; and, corona discharges. Of the four electrostatic discharges, the spark, the brush and the propagating brush electrostatic discharges can all create incendiary discharges. The corona discharge is not known to create incendiary discharges for common flammable atmospheres.
By incorporating certain materials into the flexible fabric container, as the electrostatic field increases, corona discharges from such materials limit the maximum field. This electrostatic field level, however, is above the 500 kV/m threshold level at which the potential for incendiary discharge first appears. Examples of this conventional approach include U.S. Pat. No. 4,207,376 (Nagayasu), U.S. Pat. No. 4,989,995 (Rubenstein), U.S. Pat. No. 4,900,495 (Lin), U.S. Pat. No. 4,997,712 (Lin), U.S. Pat. No. 5,116,681 (Lin) and U.S. Pat. No. 5,147,704 (Lin).
Yet another approach to the problem of incendiary discharge has been to decrease the surface resistivity of a container by coating the container with an antistatic material. Such a coating on the container surface increases the threshold level of the potential for an incendiary discharge to about 1500 kV/m. However, the potential for an incendiary discharge is still a very real possibility. Examples of this approach include U.S. Pat. No. 5,151,321 (Reeves), U.S. Pat. No. 5,092,683 (Wurr) and U.S. patent application Ser. No. 08/139,113, which is incorporated herein by reference in its entirety.